Electromagnetic transmitter



NOV. 4, 1930. A, THOMAS 1,780,409

ELECTROMAGNETIC TRANSMITTER Filed Feb. 10, 1928' INVENTOR Patented Nov. 4, 1930 UNITED? STATES ADOLIH THOMAS, OF NEW YORK, N. Y.

ELECTROMAGNETIC TRANSMITTER Application filed February 10, 1928. Serial No. 253,249.

The present invention relates to electromagnetic devices for converting mechanical ibrations into electric impulses, and its object is'to provide a transmitting instrument of that class for telephonic and phonographic use. sesses special advantages as an electric pickup, particularly for playing finely grooved vertical-cut records like those of the Edison type.

One feature of this invention is an armature consisting of a pair of oppositely bowed springs connected at the ends andspaced at the center so as to form an ellipse. One of the springs is attached at or near its middle point to a fixed support, which in some instances may be the central. pole piece of a three-pole magnet. An actuating member is connected to the middle of the other spring. The connected ends of the two springs terminate near the poles of a bipolar magnet or near the two outer poles of a three-pole magnet. As the actuating member vibrates, it varies the two .airgaps between the armature ends and the adjacent pole pieces, thereby inducing current impulses in a coil or coils operatively associated with the magnet. The oppositely curved armature springs may in some instances be formed as an integral unit in the shape of a flat ellipse.

When my invention is in the form of an electric pickup for vertical-cut records, the stylus member is preferably mounted directly on the lower armature spring, so that the smallest vibrations of the stylus point are transmitted directly to the armature without the possibility of lost motion. This makes an exceedingly sensitive pickup, particularly for the new style of electrically made records in which the spiral track runs to several hundred lines to the inch.

To explain my invention so that those skilled in the art may fully understand and practice the same. I shall describe in detail the embodiments shown in the accompanying drawings, in which- Fig. 1 is an interior slde View of an electrlc phonograph reproducer constructed 1n accordance with my invention;

Fig." 2 is'a transverse section on line 2-2 As will appear later, my invention posof Fig. l, with the cover on the outer casing;

Fig. 3 is an interior side View of a modification using a. three-polar magnet;

t represents a three-polar construction similar to Fig. 3, but with a different arrangement of the magnetic poles and 5 shows an embodiment in which a one-piece spring armature in the shape of a flat ellipse is connected to the central pole or" a three-polar magnet structure.

Before proceeding with a detailed description of the several views, l wish to explain that the drawings: have been made largely diagrammatic and the parts have been purposely exaggerated in their relative PIOPOI'". tions in the interest of clearness. The actual device is considerably smaller than the illustrations in the drawings, which are therefore not to be taken as shop drawings made to accurate scale. Also, to simplify the drawings I have shown a pickup for playing vertical-cut records, but this is not to be taken as a restriction or limitation of my invention.

Referring to Figs. 1 and 2, there is a casing of non-magnetic material, which may be brass, aluminum or molded composition, such as bakelite, For compactness and neatness of appearance, the casing may be in the form of a flat oval or ellipse, one side of which is closed by a removable cover 12. The body of the casing maybe made in a single piece. Screws 13 or the like enter screwthreaded holes in bosses 14 for holding the cover 12 in place. A hub15 projecting from one edge of the casing permits the same to be easily mounted on a supporting arm 16, which may be a small light tube supported at its rear end to allow movement of the reproducer over a record.

A permanent U-shaped magnet 17 is rigid- 1y mounted in casing 10 in any practical way, as by means of screws or bolts 18 entering bosses 19 at the top of the casing. The mag net 17 is a flat bar of good magnetic steel, such as cobalt steel or silicon steel, and may either be solid or consist of laminations held together in a rigid structure. The magnet 17 can, of course, be an electromagnet, but it is simpler to have a permanent magnet.

A cross-piece or bridge 20 of non-magnetic material is secured to the limbs of magnet 17 by screws 21 or otherwise. The bridge 20 is preferably made of aluminum on account of its strength and lightness, but it may also consist of brass, bakelite and other non-magnetic material having the requisite strength. The bridge 20 has a central projection 22, on Which is mounted a vibratory armature A consisting of a pair of oppositely bowed springs 23 and 24 connected at their ends by rivets 25 or other fastening members, or by brazing, welding, soldering, and so on. A screw 26 or the like secures the spring armature A to the bric ge extension 22, so that the armature may vibrate in its own plane. This connection is such that the armature cannot turn about the member 28 as a pivot, and yet allows free movement of the two springs in their common plane. The armature springs are thin bands of steel or any elastic magnetic material capable of responding instantly and correctly to the smallest pressure variations, as will presently be explained.

As previously stated, I have assumed that the constructions here shown were designed to play vertical-cut records of the Edison type, and so the lower armature spring 24 carries a stylus member 27 having a stylus point adapted to operate in the vertical-cut spiral groove of a record diagrammatically indicated at 28. Edison records are usually played with a diamond point, but I want it understood that I do not commit myself to any particular form of record-playing stylus. A screw or bolt 29 rigidly connects the stylus member 27 to the lower armature spring 24 at or near the center thereof. As seen in Fig. 1, the connecting point 29 is diametrically opposite the supporting point 26. The lower edge of casing 10' has an opening 30 through which the stylus member 27 projects,

The operation of the reproducer above described is this: As the stylus member 27 vibrates vertically while tracking the verticalcut spiral groove of a rotating record, the bowed armature spring 24 alternately moves up toward the fixed connecting point 26 when the stylus member 2'? rises and automatically recoils from the point of support when the stylus member descends. The weight of the reproducer when playing a record is sufficient to place the armature springs 23 and 24 under such tension that they want to move apart at their oppositely curved centers. Now, it is evident that when the armature springs are pushed together at the center along a line connecting the points 26 and 29, the connected ends 31 move outwardly toward the adjacent pole pieces 32 of the magnet 17. lVhen the armature springs are allowed to move apart at the center by their own inherent tension, the armature ends 31 recede from the pole pieces 32-. It is thus clear that the vibratory movements of the armature A in its own plane cause corresponding variations in the airgaps 33, whereby current impulses are induced in the coils 34 mounted on the polar ends of the magnet.

Only one coil is really necessary, but I prefer to use two coils connected in series. The leads for the coils may pass through the hollow supporting arm 16, so as to be concealed thereby. The coils 34 are machinewound, and after being slipped on the magnet they are secured in place in any practical way, as by cross-pins 35, or otherwise. The upper ends ofthe coils may abut against the bridge 20. One or both coils could be mounted on the upper armature spring 23, but that would not be so convenient, since the central opening of the coil would have to be curved to allow sufiicient movement of the armature. Then, again, the coils could be mounted in such a way that the vibratory armature ends 31 would extend through the coils. In the broader aspect of my invention, any suitable electromagnetic means may be used for converting the movements of the armature into electric impulses.

Fig. 3 shows a reproducer in which the magnetic field structure consists of a pair of magnets 36 and 37 rigidly connected to form a middle pole piece 38. The outer pole pieces 39 of this three-pole magnet structure are of one polarity (marked N in the drawing), and the middle pole piece 38 is of the opposite polarity (indicated by S). Bolts 40 or other fastening members hold the magnet structure 3637 in casing 10. The upper armature spring 23 carries a magnetic bracket 41, which is'mountcd on the end of the middle pole piece 38, as by a bolt or rivet 42. In this way the armature is magnetically connected to and supported by the central pole piece 38.

he screw or rivet 43, which secures the armature spring 23 to the U-shaped bracket ell, may be regarded as the fixed supporting point about which the armature vibrates in its own plane. If necessary, a fastening member A may be used to hold the magnets 36 and 3? together at the top. In this case, the coil mounted on the central pole piece 38, although a coil may be mounted on one or both pole pieces 39. The lower end of coil 34 rests on the bracket 41, and a cross-pin 4-5 holds the coil firmly in place. W hat has been said about the operation of the armature in Fig. 1, applies fully to Fig. 3 without the need of repetition.

Fig. 4 shows a three-polar magnet system in which two permanent magnets 46 and 47 are magnetically and mechanically connected along their inner polar limbs 1-6 and l7, which are of opposite polarity and therefore constitute a neutra pole piece or the neutral point of the magnetic system. The armature A is attached to the neutral pole-piece 46'-47 in the same way the armature is attached to the pole piece 38 in Fig. 3, so that I need not repeat that part of the description.

In this instance the coils 34 are mounted on the outer pole pieces of the magnets 46 and 47, being held thereon by cross-pins 35 and 48 or in any other practical way. In Fig. 4 I have purposely omitted the casing 10 because it is evident from Figs. 1, 2 and 3 how the magnets and 47 and all parts carried there by are mounted inoperative position in the Since the armature A in Fig. 4 operates in the same way as the armature in Figs. 1 and 3, I need not repeat what was explained in connection with the operation of Fig. 1.

It will be noticed that in Figs. 1 and 3 the magnetic pole pieces extend below the armature ends 31. This is to allow vertical movement of the armature ends without causing them to leave the magnetic field at the polar faces. As the bowed armature springs 23 and 24 are pushed together or pulled apart at the center, the polar ends 31 move toward and from the adjacent pole pieces, as previously explained, but there will also be acertain amount of vertical movement of the armature ends. By extending the polar end-s of'the magnets below the armature ends, the vertical movements of the latter will not carry them outside the field flux at the polar faces, so that only the horizontal movements of the arinature'toward and from the pole pieces are effective in producing electric impulses in the coil or coils 34. If desired, however, the polar ends of the magnets which form the airgaps 33 with the armature may be so arranged. that, when the reproducer is in playing position on a record, the pole pieces and adjacent armature ends are approximately in horizontal alignment, as shown in Fig. 4. In this arrangement, when the arma ture ends 31 move below the adjacent polar ends, the flux through the coils 34 is varied. In addition to that, the horizontal movements of the armature ends 31 toward and from the magnetic poles will cause flux variations through the coils. In other word-s, in a con struction like that of Fig. 4, the current impulses are produced conjointly by the horizontal and vertical movements of the armature ends 31, whereas in Figs. 1 and 3 the flux variations through the coils 34 are due almost entirely to the horizontal movement of the armature ends 31 toward and from the ad jacent pole pieces.

Fig. 5 shows a three-polar magnet struc ture similar to that of Fig. 3, in that two magnets 49 and 50 are connected to form a central pole piece 51, which is here marked as being of south polarity, while the two outer pole pieces 52 are of north polarity.

Practically the only difference between Figs. 3 and 5 is that in Fig. 5 the armature A consists of a pair of oppositely bowed springs 23' and 24' formed integral as an elastic flat ellipse. The opposite ends 53 of the elliptic armature A terminate closely to polar extensions 54 on pole piece 52. The polar extensions 54 are not absolutely necessary, but it is better to have them so as to provide curved polar faces 55 conforming to the curved ends 53 of the armature A. As the armature vibrates in its own plane, due to the vertical movement of the stylus member 27, as fully described in connection with Fig. 1, the armature ends 53 approach and recede from the, polar faces 55, thereby producing current impulses in coil 34 mounted on the middle pole piece 51. The elliptic armature A is mounted on pole piece 51 in practically the same way as armature A in Fig.3. As Fig. 5 is intended to be diagrammatic for the sake of clearness, I have omitted the casing 10.

A few words might be said about the flux paths in the different modifications. In Fig. 1 the field flux passes through both armature springs 23 and 24, which constitute parallel paths between the pole pieces 32. In Fig. 3 the field flux passes from the outer pole pieces 39 through the'armature spring 23 into the central pole piece 38. In this case the lower armature spring 24 will carry practically no flux, since the shortest path for the magnetic lines is through the upper spring 23. In Fig. 4-the three-pole magnet structure provides a magnetic path of unusual length, the flux passing through the two magnets in series and the armature springs bridging the outer pole pieces 46 and 47 in parallel paths. It is also possible that a portion of the field flux will pass from pole piece 46 through the left half of armature spring 23 into the pole piece 46'. In the same way some of the flux of magnet 47 may find a path from pole piece 47 through the right half of armature spring 23 into pole piece 47. Whichever path or paths the field flux in Fig. 4 may take, the coils 34 are so mounted as to be influenced by all flux variations. In Fig. 5 the flux path is the same as in Fig. 3.

It is hardly necessary to add that the armatures A and A are made of magnetic spring metal having the right degree of flexibility and so dimensioned as to carry the field flux without saturation. The precise dimensions of the armature springs in any particular case can be determined by experiments well within the skill of the electrical engineer. Since the stylus member 27 is directly connected to the elastic armature, the movements of the stylus point are transmitted'to the armature without lost motion, so that the instrument is exceedingly sensitive even when the stylus vibrations are microscopic in amplitude. On this account my invention is particularly useful as a re producer for the new style of long-playing records in which the needle track runs to several hundred lines to the inch. The stylus member 27, instead of being directly mounted :2 i

on the armature, may be mounted on one end of a pivoted lever which has its other end connected to the armature. For playing lateral-cut records, a pivoted needle-arm is connected to (or otherwise actuates) the center of armature spring 24, as will be understood without further illustration. The weight of the reproducer is such that the right amount of load or resistance is imposed on the armature springs.

The magnet structure in this instrument need not be so heavy as in prior pickups using a pivoted armature, where there is always lost motion and where the low efficiency of the instrument requires an unusually heavy magnet. Those heavy pickups are also objectionable because they produce loud needle noises and tend to slow up the turntable. In this case, the magnet can; be made from a flat steel bar of lightweight, and for the three-polar construction of Figs. 3, 4 and 5 two small light magnetic bars are joined together. Attention called to the fact that the three-polar magnet system pro vides a magnetic path of unusual length, thereby increasing the life of the magnetic field and the sensitivity of the instrument. The parts that go to make up my new instrument are few and cheaply made, are easily assembled and not liable to get out of order. The magnet and the parts carried thereby inserted as a unit in the casing, where they are fully exposed for easy access by simply removing the cover 12. It is thus seen that F I provide a transmitter of unusual siinplicit y and efliciency.

By making the armature loop flat or elongated in a direction at right angles to the actuating force applied to it, the flexibility of the armature is increased and that means increased sensitiveness of the instrument, because the armature is capable of responding efiiciently to the feeblest movements of the stylus member or other actuating element. In those forms of my invention where the fiat armature loop consists of two leaf springs connected at their ends, the armature is provided with polar extensions of double thickness and these extensions terminate in flat polar faces adjacent the flat polar faces of the magnet structure. This produces airgaps of appreciable cross-sectional area whereby the sensitiveness and efliciency of the device are increased. 7

Should the armature ends 31 or 53 accidentally strike the adjacent pole pieces (as when, for instance, the pickup is dropped on a record), the inherent tension of the armature springs will pull the ends away from the magnet poles the moment that the instru ment is lifted. In the normal operation of the device, such contact betweenarmature and magnet will not occur, but if it does happen by accident, the armature instantly recoils to re-establish the airgaps 33.

Although I have shown my invention in the form of an electric pickup for playing vertical-cut records, it is evident that in the broad aspect of my invention the member 27 may be an acoustic diaphragm, a needle arm for playing lateral-cut records, or any other member having movements intended to be converted into electric impulses. I want it therefore understood that the illustrations in the drawings do not impose any restrictions or limitations on my inventions unless so specified in the claims.

I claim as my invention:

1. In an electric phonograph reproducer, a pair of oppositely bowed magnetic leaf springs connected at their ends and constituting a closed flat loop, the distance between said ends being greater than the distance between the opposed central points of the leaf springs, means for supporting one of said bowed springs at the center, a stylus member connected to the central portion of the other bowed spring for vibrating both springs in their own plane, and electromagnetic means in operative relation to the ends of said leaf springs for converting the movements of said springs into electric impulse 2. An electric phonograph pickup comprising a magnet structure, a vibratory armature in the form of a closed fiat loop consisting of a pair of oppositely bowed leaf springs connected at their ends, the length of said armature between its ends being greater than the distance between the central points of said leaf springs, means for supporting one of said bowed springs at the center between two poles of said magnet structure, the connected ends of said springs forming a pair of variable airgaps with said last-named poles, a stylus member connected to the center of the other bowed spring for vibrating said armature in its own plane and thereby varying said airgaps, and a coil for converting the movements of said armature into electric impulses.

3. An electric phonograph reproducer for playing vertical-cut records, comprising a vertically movable armature in the form of a closed fiat loop consisting of a pair of oppositely bowed leaf springs connected at their ends, the distance between the ends of said armature being greater than the distance between the central points of said springs, means for supporting the upper spring at the center, a vertically movable stylus member connected to the central portion of the lower spring. and electromagnetic means for converting the movement of said armature into electric impulses.

4. An electric phonograph reproducer for playing vertical-cut records, comprising a vertically movable armature in the form of a closed flat loop consisting of a pair of oppositely bowed leaf springs connected at their ends, the distance between the ends of said armature being greater than the distance between the central points of said leaf springs, means for supporting the upper spring at the center, a vertically movable stylus member mounted on the central portion of the lower spring and supported thereby, so that the vibrations of said stylus member are directly transmitted to the armature, and electromagnetic means in operative relation to the ends of said leaf springs for converting the movement of said armature into electric impulses.

5. An electric phonograph reproducer coniprising a magnet structure, a vibratory armature in the form of a closed fiat loop consisting of a pair of oppositely bowed leaf springs connected at their ends, the distance between the ends of said armature being greater than the distance between the central points of said leaf springs, means for supporting one of said bowed springs at the center between two pole pieces of said magnet structure, the connected ends of said springs forming a pair of variable airgaps with said pole pieces, a stylus member connected to the center of the other bowed spring for vibrating said armature in its own plane and thereby varying said airgaps, and one or more coils associated with said magnet structure for converting the movements of said stylus member into electric impulses.

6. An electromagnetic transmitter comprising a three-pole magnet structure, a magnetic member removably secured to the middle pole piece of said magnet structure, a vibratory armature in the form of a closed flat loop consisting of a pair of oppositely bowed leaf springs connected at their ends, means for rigidly connecting one of said bowed springs at the center to said magnetic member, the connected ends of: said springs extending toward the two outer poles of said magnet structure and forming therewith a pair of variable airgaps, actuating means connected to the center of the other bowed spring for vibrating said armature in its own plane and thereby varying said airgaps, the length of said armature between its polar ends being greater than the distance between the centers of said bowed springs, and one or more coils mounted on said magnet structure for converting the movements of said armature into electric impulses.

7. An electromagnetic transmitter comprising a pair of substantially U-shaped magnets having two polar limbs connected together to form a middle polar projection, the other polar limbs constituting a pair of outer polar projections, a spring armature in the form of a closed flat loop having oppositely curved central portions relatively movable to permit vibration of the armature in its own plane, means for connecting one of said curved portions at the center to said middle polar projection, so that the opposite ends of said armature extend toward said outer polar projections to form therewith a pair of variable airgaps, actuating means connected to the center of the other curved portion of said armature for vibrating the same and thereby varying said airgaps, and a coil or coils for converting the movements of said armature into electric impulses.

'8. An electric phonograph reproducer for playing vertical-cut records, comprising a magnet structure, a vibratory armature consisting of a pair of oppositely bowed leaf springs arranged vertically and connected at their ends, said leaf springs forming a closed flat loop in which the horizontal diameter between the ends is greater than the vertical diameter connecting the central portions of the springs, means for supporting the upper spring at the center between two pole pieces of said magnet structure, the connected ends of said springs forming a pair of variable airgaps with said pole pieces, a vertically movable stylus member mounted on the centralportion oi the lower spring, whereby the vibrations of said stylus member are directly transmitted to the armature springs to vibrate the latter in a vertical plane and vary said airgaps, and one or more coils associated with said magnet structure for converting the movements of said stylus member into electric impulses.

9. An electric phonograph reproducer comprising a pair of substantially U-shaped magnets having two polar limbs connected together to form a middle polar projection, the other polar limbs constituting a pair of outer polar projections, a spring armature in the form 01": a closed fiat loop having oppositely curved central portions relatively movable to permit vibration of the armature in its own plane, means for connecting one of said curved portions at the center to said middle polar projection, so that the opposite ends of said armature extend toward said outer polar projections to form therewith a pair of variable airgaps, the distance between the polar ends of said armature loop being great er than the distance between said curved central portions, a stylus member connected to the center of the other curved portion of said armature for vibrating the same and thereby varying said airgaps, and one or more coils associated with said magnets to produce current impulses in accordance with the vibrations of said stylus member.

10. An electric phonograph reproducer for playing vertical-cut records, comprising a three-pole magnet structure, a spring armature in the form of a closed-fiat loop having oppositely curved central portions arranged vertically and relatively movable to permit vibration of the armature in a vertical plane, means for connecting the upper spring portion at the center to the middle pole piece of said magnet structure, so that the opposite ends of the armature extend toward the two outer pole pieces of said magnet structure to form a pair of airgaps therewith, a vertically movable stylusmember mounted on the 6 lower spring portion at the center thereof,

whereby the vibrations of said stylus member are directly transmitted to said armature for vibrating the same in a vertical plane and thereby varying said airgaps, and one or 10 more coils associated with said magnet structure to produce current impulses in accordance with the vibrations of said stylus member.

11. An electromagnetic transmitter comprising a pair of substantially U-shaped magnets having two polar limbs of like polarity magnetically connected to form a middle polar projection, the other polar limbs constituting a pair of outer polar projections, a $9 spring armature in the form of a closed fiat loop having oppositely curved central portions relatively movable to permit vibration of the armature in its own plane, means for connecting one of said curved porg5 tions at the center to said middle polar projection, so that the opposite ends of said armature extend toward said outer polar projections to form therewith a pair of variable airgaps, actuating means connected to the center of the other curved portion of said armature for vibrating the same and thereby varying said airgaps, the diameter of said armature loop between its polar ends being greater than the distance between said curved central portions, and a coil mounted on said middle polar projection for converting the movements of said armature into electric impulses.

12. In an electromagnetic instrument of the telephone type, a vibratory magnetic member consisting of a pair of oppositely bowed leaf springs connected at their ends, which constitute polar extensions having approximately twice the thickness of a single spring, 4r said extensions terminating in substantially fiat polar faces, means for supporting one of said springs at the center, the distance between said polar extensions being greater than the distance between the central points of said springs, and magnetic poles adjacent said flat polar faces of the leaf springs.

ADOLPH A. THOMAS. 

